Calculations of free energy barriers for local mechanisms of hydrogen diffusion in alanates

Abstract

Brute force histogram calculation and a recently developed method to efficiently reconstruct the free energy profile of complex systems ( the single-sweep method) are combined with ab initio molecular dynamics to study possible local mechanisms for the diffusion of hydrogen in sodium alanates. These compounds may help to understand key properties of solid state hydrogen storage materials. In this work, the identity of a mobile species observed in experiments characterizing the first dissociation reaction of sodium alanates is investigated. The activation barrier of two suggested processes for hydrogen diffusion in Na(3)AlH(6) is evaluated and, by comparing our results with available experimental information, we are able to discriminate among them and to show that one is compatible with the observed signal while the other is not

Synthesis of calcium alanate and its dehydriding performance enhanced by FeF3 doping

“…It is interesting to see that the activation energy tends to be higher with the increasing of the size of metal cation. The value of CaAlH 5 is much higher than those of K 3 AlH 6 and Na 3 AlH 6 which are technically reversible [8,24], this indicates that mass transport is much more difficult in the dehydrogenation and rehydrogenation processes of CaAlH 5 . Fortunately, FeF 3 -doped sample exhibits favorable value compared to those of doped K 3 AlH 6 and Na 3 AlH 6 .…”

“…As-prepared 120 [24] Doped 97-100 [24] for the as-prepared and FeF 3 -doped samples. It is observed that the activation energy of the as-prepared sample is 153.4 kJ/mol, which is in good agreement with the published result in other literature [15].…”

p of silicic acid in presence of La3+using single sweep method coupled to DFT-based molecular dynamics

“…Derivation and details of single sweep method to obtain free energy curves are reported in the seminal works of Vanden-Eijnden, Ciccotti and co-workers [18,19,24]. Here, after describing the system, we briefly summarise the main points of the method and report how we have applied it to our particular case.…”

“…For each pK a calculation, we have first performed TAMD [19,24] simulations to span values of the O-H distance and thereby generate initial conditions for the free energy curve reconstruction performed in a second stage. In the TAMD simulation, an extended Lagrangian is used to couple the physical order parameters to an additional fictitious variable, itself having a fictitious mass, by means of a harmonic coupling.…”

“…For example, in in the past years Sprik and co-workers have largely examined the reliability of DFT-based dynamics to calculate pK a of different molecules: first by using constrained dynamics to build free energy curves along the protonation/deprotonation reaction pathways [14] and more recently by a reversible insertion/removal of protons method [15][16][17] Among methods aimed to overcome barriers and thus compute the free energy curves along a reaction pathway, Vanden-Eijnden [18] developed the single sweep method. Subsequently, Vanden-Eijnden, Ciccotti and their co-workers have shown how this method is suited to be used in different systems and in particular in conjunction with DFT-based dynamics [19][20][21][22][23][24]. For that reason, we have applied this method to the determination of pK a of a weak acid in presence of a highly charged cation to show how and to which extent the metal decreases the pK a of the acid.…”